The figures are for PM2.5 or particulate matter tiny enough to be absorbed into human tissue.

Air quality has long been a problem in the Chinese capital, but this
weekend saw levels more than 30 times above the level judged safe by the
World Health Organisation.

Breakneck economic growth, reliance on
coal, dramatic expansion of car ownership and the widespread flouting
of environmental laws have all contributed to China's
air pollution problems. But the intensity of the current problem
appears to be weather-related. The monitoring centre said the heavy
pollution had been trapped by an area of low pressure and warned that
the problem was likely to continue until Tuesday.

It's bad enough that Beijingers have to exist in a particulate fog but that doesn't begin to explore just what those micro-particles being absorbed into their bodies actually contain. China is known to produce some truly wicked air pollution. Lots of heavy metals, acids, toxins including arsenic and carcinogens.
Soil pollution is another enormous problem facing China which accounts for 70% of the world's arsenic pollution. It's been estimated that up to 40% of China's farmland is now too contaminated by airborne arsenic to be capable of producing crops fit for human consumption.